Description

Sir Alonne came to this land from the east, chose to serve a little -known and unestablished lord, and helped him become the Old Iron King.
Then, at the very peak of his sire's rule, Sir Alonne set out again, in search of land yet unknown.1

Attack Patterns

A downwards diagonal slash, followed quickly by a more weighted slash back up.

A jump backwards and then a thrust.

A jump backwards and then an upward swing.

When at long range, his sword will glow, and he will fire a Dark projectile.

Kick, low damage.

His sword will glow, and he will do a thrust. If it hits he will become buffed the same as the Bewitched Alonne Sword.

It does not work on NPC summons.

After 60% health; a jumping downward plunge, usually followed by ~2 seconds of not moving, though rarely he will immediately do a horizontal swing. Has a small area of effect explosion.

When he holds his sword low to the ground, he can parry your attacks and follow up with a riposte.

Strategies

The key to this fight is to learn when to dodge his charging attacks, which have wildly different speeds. When he is lowering his blade to prepare for a dash attack, if he lowers it slowly, then there will be a slightly longer delay before his dash.

After he jumps away from you to do a charge (holds his sword horizontally) it is best to roll backwards. Afterwards quickly close the gap and you have a prime opportunity to do damage.
When he does his two diagonal slashes, if you roll forward and right on the first, the second should go over your head without needing to roll; saving stamina for a counterattack.

He will occasionally attempt to impale you (telegraphed by his sword glowing); roll towards or sideways to avoid. If this move connects, he'll buff his weapon with the same effect as the Bewitched Alonne Sword buff.

Be careful of getting too close to the wall while locked on, as if you move in front of a pillar the camera will move upwards and point into the ground, making it difficult to see.

Every time you dodge his attack, try to roll forward and slightly to the right, then hug his back side. His attacks become much less efficient when you're behind him (or behind his left side).

There are several points in the room where if Sir Alonne loses vision of you, he will no longer attack you or make any moves. If he is backed into either of the two corners on the side of the room with the entrance, standing on the other side of the pillar closer to the door such that you can't see him will give you a chance to heal, cast miracles, or even re-equip. Take note that if he isn't in the corner and you can still see the edge of his sword, he will still try to hit you through the wall, some of these attacks can hit you, so be sure you can't see him at all.

Notes

If you manage to kill him without taking damage, he performs Seppuku. One can try this by quitting and reloading upon getting hit, as the player will just appear outside the boss fog upon reloading. That aside, there is no special reward for defeating him this way. The Seppuku animation is invalidated if you took self-damage, like from the Chaos Blade for example.